Taming Heavier Group 14 Imine Analogues: Accessing Tin Nitrogen [Sn=N] Double Bonds and their Cycloaddition/Metathesis Chemistry

Abstract A systematic study to access stable stannaimines is reported, by combining different heteroleptic stannylenes with a range of organic azides. The reactions of terphenyl‐/hypersilyl‐substituted stannylenes yield the putative tin nitrogen double bond, but is directly followed by 1,2‐silyl migration to give SnII systems featuring bulky silylamido ligands. By contrast, the transition from a two σ donor ligand set to a mixed σ‐donor/π‐donor scaffold allows access to three new stannaimines which can be handled at room temperature. The reactivity profile of these Sn=N bonded species is crucially dependent on the substituent at the nitrogen atom. As such, the Sn=NMes (Mes=2,4,6‐Me3C6H2) system is capable of activating a broad range of substrates under ambient conditions via 1,2‐addition reactions, [2+2] and [4+2] cycloaddition reactions. Most interestingly, very rare examples of main group multiple bond metathesis reactions are also found to be viable.


Synthesis of Mes TerSnN(Mes)Si(SiMe3)3 (Sn3b)
To a solution of Mes TerSnSi(SiMe3)3 (Sn2) (0.030 g, 0.044 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.007 g, 0.044 mmol) in 0.3 mL of C6D6 which results in immediate gas evolution and a colour change to orange. All volatile components were removed under vacuum and the residue was dissolved in 1 mL of n-hexane, filtered and either stored at -30 °C to give Mes TerSnN(Mes)Si(SiMe3)3 (Sn3b) as an orange crystalline solid or followed by removal of all volatile components to give Sn3b as a powder. The supernatant was removed via syringe and the crystals dried under vacuum. Crystals obtained this way were suitable for single crystal X-ray diffraction.

Synthesis of Mes TerSnN(Dipp)Si(SiMe3)3 (Sn3c)
To a solution of Mes TerSnSi(SiMe3)3 (Sn2) (0.040 g, 0.059 mmol) in 0.3 mL of C6D6 was added a solution of N3Dipp (A3) (0.012 g, 0.059 mmol) in 0.3 mL of C6D6 which results in immediate gas evolution and a colour change to red. All volatile components were removed under vacuum and the residue was dissolved in 1 mL of n-hexane, filtered and either stored at -30 °C to give Mes TerSnN(Dipp)Si(SiMe3)3 (Sn3c) as dark orange-red crystals or followed by removal of all volatile components to give Sn3c as a powder. The supernatant was removed via syringe and the crystals dried under vacuum. Crystals obtained this way were suitable for single crystal X-ray diffraction.

Attempted Reaction of Mes TerSnSi(SiMe3)3 (Sn2) with N3 Mes Ter (A4)
To a solution of Mes TerSnSi(SiMe3)3 (Sn2) (0.020 g, 0.029 mmol) in 0.3 mL of C6D6 was added a solution of N3 Mes Ter (A4) (0.011 g, 0.029 mmol) in 0.3 mL of C6D6. Since subsequent NMR analysis revealed no reaction, the reaction mixture was heated to 80 °C for 24 h which also did not result in any reaction between both reactants. Shown below is the 1 H NMR spectrum of pure N3 Mes Ter (A4) and the obtained 1 H NMR spectrum of the reaction mixture after 24 h at 80 °C.

Synthesis of Mes TerSn(N(SiMe3)2)2N3 (Sn6)
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.050 g, 0.084 mmol) in 0.3 mL of C6D6 was added a solution of N3SiMe3 (A1) (0.010 g, 0.084 mmol) in 0.3 mL of C6D6. Subsequent NMR analysis of the reaction mixture revealed fast consumption of A1 with approximately 0.5 equivalents of Sn1 remaining unreacted. Therefore, additional amounts of A1 was added leading to full consumption of Sn1 and formation of Mes TerSn(N(SiMe3)2)2N3 (Sn6). All volatile components were removed under vacuum and the slightly yellow solid was dissolved in 0.7 mL of n-hexane, filtered and stored at -30 °C to give Sn6 as colourless crystals. Crystals obtained this way were suitable for single crystal X-ray diffraction.  C,54.39;H,7.73;N,8.81;Found: C,55.11;H,7.81;N,8.21.

Synthesis of Mes TerSn(N(SiMe3)2)=NMes (Sn7a) and of the C-H-Activation Product Sn4b
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.040 g, 0.068 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.011 g, 0.068 mmol) in 0.3 mL of C6D6 leading to immediate gas evolution and a colour change to orange-red. Subsequent NMR analysis revealed clean formation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). Removal of all volatile components yielded Sn7a as an orange-red solid. Dark orange-red crystals of Sn7a suitable for single crystal X-ray diffraction were obtained from a saturated n-hexane solution of Sn7a at -30 °C. In solution Sn7a undergoes intramolecular C-H-activation to give Sn4b. Therefore, only the 1 H NMR data of Mes TerSn(N(SiMe3)2)=NMes (Sn7a) is given below. To obtain analytically pure Sn4b, the above-mentioned reaction mixture was heated to 70 °C for 16 h resulting in a colour change to pale yellow. All volatile components were removed under vacuum. The residue was suspended in 0.7 mL of n-hexane, filtered and stored at 4 °C to give Sn4b as colourless crystals. Crystals obtained this way were suitable for single crystal X-ray diffraction.
Note: Mes TerSn(N(SiMe3)2)=NMes (Sn7a) even in the solid state slowly undergoes intramolecular C-H-activation to give Sn4b. Due to this behaviour, Sn7a was synthesized in situ and directly reacted further for the reactivity studies. Although Sn7a can be obtained repeatedly as a single crystalline material following the above procedure, we recommend to follow the in situ protocol.

Synthesis of Mes TerSn(N(SiMe3)2)=NDipp (Sn7b) and of the C-H-Activation Product Sn4c
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.050 g, 0.084 mmol) in 0.3 mL of C6D6 was added a solution of N3Dipp (A3) (0.017 g, 0.084 mmol) in 0.3 mL of C6D6 leading to immediate gas evolution and a colour change to orange-red. Subsequent NMR analysis revealed clean formation of Mes TerSn(N(SiMe3)2)=NDipp (Sn7b). Removal of all volatile components yielded Sn7b as an orange-red solid. In solution Sn7b undergoes intramolecular C-H-activation to give Sn4c. Therefore, only the 1 H NMR data of Mes TerSn(N(SiMe3)2)=NDipp (Sn7b) is given below. To obtain analytically pure Sn4c, the above-mentioned reaction mixture was heated to 70 °C for 16 h resulting in a colour change to pale orange. All volatile components were removed under vacuum. The residue was suspended in 0.7 mL of n-hexane, filtered and stored at 4 °C to give Sn4b as colourless crystals. Crystals obtained this way were suitable for single crystal X-ray diffraction.

Attempted Reaction of Mes TerSnN(SiMe3)2 (Sn1) with N3 Mes Ter (A4)
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.030 g, 0.051 mmol) in 0.3 mL of C6D6 was added a solution of N3 Mes Ter (A4) (0.018 g, 0.051 mmol) in 0.3 mL of C6D6. Since subsequent NMR analysis revealed no reaction, the reaction mixture was heated to 80 °C for 24 h which also did not result in any reaction between both reactants. Shown below is the 1 H NMR spectrum of pure N3 Mes Ter (A4) and the obtained 1 H NMR spectrum of the reaction mixture after 24 h at 80 °C.

Synthesis of Mes TerSn(hmds)=NQuin (Sn8) and of the C-H-Activation Product Sn4d
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.025 g, 0.042 mmol) in 0.3 mL of C6D6 was added a solution of N3Quin (A5) (0.007 g, 0.042 mmol) in 0.3 mL of C6D6. The reaction immediately results in gas evolution and a colour change to dark green. Subsequent NMR analysis ( Figure  S40) reveals overall clean conversion to a single product. All volatile components were removed under vacuum and the residue was suspended in 0.5 mL of n-hexane, filtered and stored at -30 °C to give dark green crystals suitable for single crystal X-ray diffraction, confirming that the initial product of the reaction is Mes TerSn(N(SiMe3)2)=NQuin (Sn8). In solution Mes TerSn(N(SiMe3)2)=NQuin (Sn8) reacts to another single product and this reaction progress was monitored by 1 H NMR spectroscopy ( Figure S37). After the reaction is finished all volatile components were removed under vacuum and the remaining solid was suspended in 0.5 mL of n-heptane. Filtration and subsequent storage of the bright yellow solution at -30 °C yields the intramolecular C-H activation product Sn4d as a crystalline yellow solid. These crystals were suitable for single crystal X-ray diffraction. The remaining small amount of crystals (approx. 5 mg) was used for multinuclear NMR spectroscopy. During one attempt to obtain crystals of Sn4d suitable for single crystal X-ray diffraction at higher temperatures (>80 °C) a small amount of red crystals was obtained suitable for single crystal X-ray diffraction and were verified to be the dimeric complex Sn9 shown in Figure S44.
Note: Mes TerSn(N(SiMe3)2)=NQuin (Sn8) in the solid state slowly undergoes intramolecular C-H-activation to give Sn4d. Due to this behaviour, Sn8 was synthesized in situ and directly reacted further for the reactivity studies.

Synthesis of Mes TerSn(N(SiMe3)2)(CCPh)NHMes (Sn10a)
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.050 g, 0.084 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.014 g, 0.084 mmol) in 0.3 mL of C6D6 for the in situ generation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). After the gas evolution has stopped, phenylacetylene (0.009 g, 0.084 mmol) was added which results in a colour change to a bright yellow-orange. All volatiles were removed under vacuum and 0.6 mL of n-hexane were added to the solid followed by filtration and subsequent storage of the saturated solution at 4 °C to give Mes TerSn(N(SiMe3)2)(CCPh)NHMes (Sn10a) as colourless crystals. Crystals obtained this way were suitable for single crystal X-ray diffraction.

Reaction of Mes TerSn(N(SiMe3)2)=NMes (Sn7a) with CO2
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.016 g, 0.027 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.004 g, 0.027 mmol) in 0.3 mL of C6D6 for the in situ generation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). The solution was freeze-pump-thaw degassed three times and backfilled with 1 bar of CO2 which results in an immediately occurring colour change from deep red to colourless. After the red colour has disappeared completely, the reaction mixture was analysed by NMR spectroscopy verifying clean formation of the [2+2] cycloaddition product Sn11a. The solution was concentrated under vacuum to approximately 0.1 mL followed by addition of 0.5 mL of n-heptane which results in precipitation of a colourless solid. The suspension was heated to 100 °C for a prolonged time which results in the formation of colourless crystals above the solution phase. These crystals were suitable for single crystal X-ray diffraction. For isolation of Sn11a, Mes TerSnN(SiMe3)2 (Sn1) (0.030 g, 0.051 mmol) in 0.3 mL of C6D6 was added to a solution of N3Mes (A2) (0.008 g, 0.027 mmol) in 0.3 mL of C6D6 for the in situ generation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). The solution was freezepump-thaw degassed three times and backfilled with 1 bar of CO2. After all starting material has been consumed according to 1 H NMR spectroscopy, the solution was transferred to a preweight vial. The vial was transferred to a Schlenk tube and the solution was carefully dried under vacuum to give Sn7a as a slightly yellow powdery material.

Synthesis of the [2+2] cycloaddition product Sn11b
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.025 g, 0.042 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.007 g, 0.042 mmol) in 0.3 mL of C6D6 for the in situ generation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). After the gas evolution has stopped, the solution was added to N,N'-diisopropylcarbodiimide (0.005 g, 0.042 mmol) which results in a colour change to bright yellow. All volatiles were removed under vacuum and 0.8 mL of n-hexane were added to the solid followed by filtration and subsequent storage of the saturated solution at -4 °C to give the [2+2] cycloaddition product Sn11b as colourless crystals. Crystals obtained this way were suitable for single crystal X-ray diffraction.
Upscale experiment: To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.500 g, 0.844 mmol) in 4 mL of benzene N3Mes (A2) (0.136 g, 0.844 mmol) in 1 mL of benzene was added and the reaction mixture was stirred until the gas evolution has stopped. An aliquot was taken (0.2 mL), dried and analysed by 1 H NMR spectroscopy to prove the purity of Mes TerSn(N(SiMe3)2)=NMes (Sn7a) ( Figure S62). To the remaining solution N,N'-diisopropylcarbodiimide (0.106 g, 0.844 mmol) in 1 mL of benzene was added and the reaction mixture was stirred for 16 h at room temperature to give a slightly yellow solution. All volatile components have been removed under vacuum to give Sn11b as a slightly yellow powdery material (corresponding 1 H NMR shown in Figure S63).

Reaction of Mes TerSn(N(SiMe3)2)=NMes (Sn7a) with CS2
To a solution of Mes TerSnN(SiMe3)2 (Sn1) (0.035 g, 0.059 mmol) in 0.3 mL of C6D6 was added a solution of N3Mes (A2) (0.010 g, 0.059 mmol) in 0.3 mL of C6D6 for the in situ generation of Mes TerSn(N(SiMe3)2)=NMes (Sn7a). An excess of CS2 (5 drops with a 1 mL syringe) was added and the reaction progress was monitored by 1 H NMR spectroscopy revealing complete consumption of Sn7a within 16 h at room temperature. The corresponding 1 H NMR spectrum ( Figure S61) revealed that significant amounts of the intramolecular C-H activation product Sn4b have been formed alongside a single other product (ratio approximately 1.5:1.0 as estimated by integration of appropriate 1 H NMR signals). All volatile components were removed under vacuum, the remaining solid was suspended in 0.7 mL of n-hexane, filtered and stored at -30 °C. Both Sn4b and the [4+2] cycloaddition product Sn12 co-crystallized. Red crystals of Sn12 were suitable for single crystal X-ray diffraction. Crystals of Sn12 were separated as good as possible and allowed for assignment of the main signals by 1 H NMR spectroscopy ( Figure S62). Due to these difficulties only the 1 H NMR data and the results of single crystal X-ray diffraction are given.

Crystallographic Details
Single crystal X-ray diffraction data for al compounds were collected at 150 K on an Oxford Diffraction/Agilent SuperNova diffractometer using Cu-K radiation ( = 1.54184 Å) or Mo-K radiation ( = 0.71073 Å), and equipped with a nitrogen gas Oxford Cryosystems cooling unit. [S5] Raw frame data were reduced using CrysAlisPro. [S6] The structures were solved using SHELXT [S7] and refined to convergence on F 2 by full-matrix least-squares using SHELXL [S8] in combination with OLEX2. [S9] Distances and angles were calculated using the full covariance matrix. Restraints were used to maintain sensible geometries for the disordered groups and approximate the displacement parameters to typical values. Selected crystallographic data are summarized in tables S2-S4 and full details are given in the supplementary deposited CIF files (CCDC 2191482-2191501). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/data_request/cif.

Computational Details
All computations were carried out using the Gaussian 16 software package or the ORCA 5.0 software package. [S12,S13] Gas phase optimizations and frequency analyses were carried out using the M06-2X functional [S14] and the def2-SVP basis set. [S15] Each system was treated with a Grimme dispersion correction with Becke-Johnson damping (GD3BJ).
[S16] The optimized structures were confirmed to be minima on the potential energy surface by the absence of imaginary frequencies. Transition states were confirmed to be local energy maxima by the presence of a single imaginary frequency along the bond forming/breaking path. Furthermore, an intrinsic reaction coordinate calculation in the forward and back directions was performed. Natural bonding orbital (NBO) analyses were carried out using the NBO 7.0 program. [S17] Atoms in molecules (AIM) analyses were conducted using the AIMAll software package. [S18] Figure S80. Selected molecular orbitals of the optimized structure of Sn7a revealing the major bonding within the SnN moiety.